PRISMSProfile Resolving In Situ Soil Moisture Sensor

The ability to acquire time series measurements of in situ soil moisture in continuous profile has long been desired in numerous applications ranging from waste site remediation monitoring to dam and levee health surveillance, early warning of landslide potential to improved irrigation management, as well as monitoring of land-atmosphere interactions for input to climate models.

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1 meter rigid PRISM probe,
installs in a hand augered hole.

Transcend Engineering has invented a sensing technology called PRISMS, for "Profile Resolving In Situ Soil Moisture Sensor," that provides long-term time series monitoring of spatially resolved profiles of soil moisture content in the vadose zone. PRISMS combines advances in the processing of TDR data with innovations in the physical sensing apparatus. These advancements enable high temporal (and thus spatial) resolution of reflectance continuously along the inside surface of a borehole using a waveguide that is permanently emplaced in contact with the adjacent soils.PRISMS is in use by pioneering researchers investigating the role of soil moisture dynamics in important terrestrial processes. Development of PRISMS has been supported through funding from the US Department of Energy's Office of Science through a Small Business Innovation Research (SBIR) grant.

PRISMS Video

Animated time series of vertical moisture profile from a PRISMS prototype showing summer drying followed by a major wetting event, at Rifle, CO.

PRISMS uses the principle of Time Domain Reflectometry (TDR) to measure soil moisture. Unlike conventional TDR, PRISMS provides detailed profiles of moisture along waveguides up to several meters in length. Conventional TDR probes only provide a single measurement of an average volume. TDR waswas originally developed to locate faults in communication and power cables. In this technique, a reflectometer sends an electrical pulse into the cable. Changes in the cable electrical impedance cause reflection of the pulsed voltage, and propagation speed of the wave is also a function of the cable impedance.Along a uniform cable, impedance is a function of the dielectric permittivity, (epsilon), and electrical conductivity (sigma) of the material surrounding the conductors.

In soil, these electrical properties are closely related to water content. TDR was adapted to estimate soil water content by Hoekstra and Delaney (1974), Topp et al (1980), and others by examining the travel time of reflection from the end of a transmission line comprising bare metal rods inserted into the soil. Advances have since led to a number of commercial TDR soil moisture probes, which until now have all used short conductors in the soil to report a single moisture value representing the average over the length of the waveguide.

Using algorithms that sort out variable wave propagation speed and multi-reflection phenomena, PRISMS takes into account the full TDR reflection profile, representing moisture changes over the entire length of a waveguide, to obtain a continuous spatial profile of moisture content.

Support provided by Vermont EPSCoR with funds from the National Science Foundation Grant EPS-1101317. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.